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00:00:00
Thank you to Brilliant for supporting00:00:01
PBS.00:00:04
>> Just over three million years ago, a00:00:06
giant ground sloth wandering through a00:00:08
tropical forest did something that00:00:10
seemed unremarkable. It kept walking.00:00:13
But these particular steps were anything00:00:15
but [music] ordinary. Because this sloth00:00:17
was doing something almost no big00:00:19
mammals had done for millions of years.00:00:21
It was walking from South America to00:00:23
North America. The land it walked on,00:00:25
the ismas of Panama, had only recently00:00:27
emerged from beneath the ocean,00:00:29
connecting the two continents after a 2000:00:30
million-year separation. And suddenly,00:00:33
South American creatures, from terror00:00:34
birds to giant sloths, found themselves00:00:36
with an entire new continent to explore,00:00:39
while North America's grumpers and00:00:40
saber-tooth cats migrated south. On00:00:42
land, the ismas kicked off possibly the00:00:45
greatest natural experiment in the00:00:46
history of life on Earth, the so-called00:00:49
Great American Biotic Interchange, or00:00:51
Gabby. But in the water, this narrow00:00:53
strip of land did something completely00:00:54
different. It divided. The ismist split00:00:57
the Atlantic and Pacific into two00:00:58
separate oceans. And that division would00:01:00
have a much bigger effect on the planet00:01:02
than the land connection ever did.00:01:04
Because this tiny strip of land probably00:01:06
changed ocean currents around the globe,00:01:08
transformed climate patterns, and even00:01:10
helped trigger an ice age.00:01:15
In the middle of the 19th century, a00:01:17
naturalist studying fish in Central00:01:18
America noticed something odd. Of the00:01:21
more than 150 species he observed off00:01:23
Panama's [music] eastern and western00:01:24
coasts, over one-third seemed to be00:01:26
identical. So the oceans must have been00:01:28
connected at some point. Not long after,00:01:30
Alfred Russell Wallace, yes, the Wallace00:01:32
line guy, noticed a pattern in American00:01:35
fossils that would later be recognized00:01:36
as evidence of the great American biotic00:01:38
interchange. So scientists began to00:01:40
piece together that an ismas had risen00:01:42
to [music] connect the continents and00:01:43
close off that once flowing seaway. And00:01:45
over the next century, as plate00:01:47
tectonics came into focus, they started00:01:49
to figure out the general picture of how00:01:51
this happened. See, after South America00:01:53
broke away from Gonduana in the late00:01:54
messoic era, it had been isolated for00:01:56
millions of years, giving rise to00:01:58
bizarre creatures like car-sized00:02:00
gipadons and those terror birds.00:02:02
Meanwhile, the Feralon plate was slowly00:02:04
subducting beneath the Caribbean and00:02:06
South American plates, triggering00:02:07
vcanism that built a chain of islands,00:02:09
the Panama Ark. And over millions of00:02:11
years, that arc crept eastward,00:02:13
eventually colliding with South America00:02:15
[music] and driving uplift that pushed00:02:17
some of the seafloor above sea level00:02:19
while also causing more volcanism that00:02:20
helped build land. Eventually, this00:02:22
combination of volcanic activity and00:02:24
tectonic collision pushed enough land up00:02:26
to form the ismas, transforming the00:02:28
Panama Ark from an island chain into a00:02:31
land bridge. But scientists still hadn't00:02:32
figured out one thing. When did it00:02:34
finally become a complete bridge, one00:02:36
that sloth could just meander [music]00:02:38
across? The first solid evidence came00:02:39
from seafloor cores in the 1970s which00:02:42
revealed that groups of tiny marine00:02:44
organisms called forammenifera began00:02:46
diverging from each other around 3 and a00:02:48
half million years ago. Then throughout00:02:50
the 1980s and into the '90s, scientists00:02:52
looked at DNA from marine species on00:02:54
both sides of Panama. They used the fact00:02:56
that DNA accumulates changes over time00:02:58
at a known rate to compare how different00:03:00
the Atlantic and Pacific populations had00:03:02
become [music] and estimate when the00:03:04
population split. This method is often00:03:06
called the molecular clock. And these00:03:07
molecular clocks also pointed to around00:03:09
3 million years ago. And the fossil00:03:11
record on land seemed to agree. There00:03:13
were tons of terrestrial vertebrates00:03:15
crossing between the continents at this00:03:16
time. So a mystery solved, right? The00:03:18
ismas closed about 3 million years ago.00:03:20
The timeline [music] made sense,00:03:22
explaining why the Gabby happened when00:03:23
it did and why the oceans changed when00:03:25
they did. For nearly 40 years,00:03:26
scientists accepted it. But then in00:03:28
2015, two studies dropped just days00:03:31
apart that completely contradicted this00:03:33
timeline. The first study compiled00:03:35
fossil records from hundreds of sites00:03:36
across both continents looking for00:03:38
patterns that previous smaller studies00:03:40
might have missed. They discovered that00:03:41
some animals hadn't waited until 300:03:43
million years ago to cross. Instead,00:03:45
there was evidence of earlier pulses of00:03:47
migration at [music] both 20 and 600:03:49
million years ago. And that same study00:03:51
found that the sea creatures backed up00:03:52
this timeline, too. Taking a wider look00:03:54
at genetic analyses, they found that00:03:56
some marine life actually began evolving00:03:58
separately around the time of those00:04:00
pulses of migration, as if something was00:04:02
already disrupting the flow between the00:04:04
oceans. The land migrations and marine00:04:06
divergences challenged the idea that the00:04:08
terrestrial groups migrated and marine00:04:10
groups split about 3 million years ago.00:04:12
The second study turned to geology,00:04:13
specifically tiny crystals called00:04:15
zirkons that form when volcanic rocks00:04:17
cool. Because here's the thing, are you00:04:19
ready? Panama's rocks have a very00:04:21
distinctive signature. They're around 5000:04:23
million years old and they formed in a00:04:25
specific volcanic environment that gives00:04:27
them a unique chemical makeup. It's like00:04:29
a geological fingerprint that's only00:04:31
found in Panama. So when the researchers00:04:32
looked at 15 million-year-old river and00:04:34
shallow marine sediments in South00:04:36
America, they found something weird.00:04:39
Zirkons with Panama's distinctive00:04:40
signature mixed in, which meant that00:04:42
rivers were already carrying Panama rock00:04:44
debris to South America 15 million years00:04:46
ago. And for rivers to be eroding Panama00:04:49
and flowing to South America, Panama had00:04:51
to be above water and connected by land.00:04:53
Both of these studies pointed to the00:04:55
idea that maybe the ismas existed as00:04:57
early as 15 million years ago, if not00:04:59
earlier, way before anyone thought. But00:05:01
scientists now faced a puzzle. If the00:05:03
ismas formed that long ago, why did most00:05:05
land animals wait millions of years to00:05:07
cross? Because it's clear that the vast00:05:09
majority of crossings happened much more00:05:10
recently. And how are marine species00:05:12
still mixing between oceans until 300:05:14
million years ago? The evidence pointed00:05:16
to two different timelines entirely and00:05:18
both couldn't be right unless scientists00:05:20
were asking the wrong questions. Many00:05:22
answering the question of when did the00:05:24
estim form required asking a more00:05:26
specific question. How exactly did the00:05:28
final closure happen? And the solution00:05:30
came from a surprising place. Looking at00:05:33
a modern example of what ancient Panama00:05:35
might once have been like, the00:05:36
Indonesian archipelago. You see, between00:05:38
the Sunda continental shelf on the Asian00:05:40
side [music] and the Sahul continental00:05:42
shelf on the Australian side sits a00:05:44
group of deep water islands called00:05:45
Waclesia. Yep. Named after [music] the00:05:47
same Wallace, Waclesia is also a00:05:49
volcanic arc that creates both a partial00:05:51
barrier and a partial connection between00:05:53
two regions. These Indonesian islands00:05:55
form a chain with gaps between them00:05:57
affecting migration on water and land in00:05:59
interesting ways. Like we talked about00:06:00
in our Wallace line episode, even narrow00:06:03
gaps between the islands with strong00:06:04
ocean currents rushing through them can00:06:06
prevent land animals from crossing. Gaps00:06:08
like the Wallace line, for example, are00:06:10
enough to confine most creatures to00:06:11
their respective sides despite being00:06:13
only about 30 km across. And that's00:06:16
because powerful ocean currents can help00:06:17
create these invisible barriers. Yet,00:06:19
some creatures do make it across, at00:06:21
least occasionally, and marine life can00:06:23
still mix through the channels between00:06:24
islands. So by 2016, scientists realized00:06:27
what would it mean if Panama didn't form00:06:29
all at once, but instead as a complex00:06:31
chain of islands first that slowly00:06:33
closed over millions of years. This00:06:35
would explain everything in the00:06:36
conflicting data. There could be early00:06:38
islands above water separated by deep00:06:40
channels that still allowed for00:06:41
submarine mixing while strong currents00:06:43
held off most of the land crossings00:06:44
until the very end. Armed with this new00:06:46
model, scientists could finally piece00:06:48
together a more detailed picture of the00:06:50
closure with a timeline to go with it.00:06:52
Most scientists now agree that 2400:06:54
million years ago, the Panama Ark riding00:06:57
on the Caribbean plate first collided00:06:58
with South America underwater. At this00:07:00
point, the Atlantic and Pacific oceans00:07:02
were still connected with deep channels00:07:04
open between them. But by sometime00:07:06
between 15 and 9 million years ago, some00:07:08
islands had pushed above sea level,00:07:10
which is why there's evidence of rivers00:07:11
from those zirkons. Deep channels,00:07:13
however, continued to separate them.00:07:15
Then by around 9 million years ago, the00:07:17
deepest passages closed and deep water00:07:19
critters became cut off while shallow00:07:21
water creatures continued to move00:07:22
across. As more and more land emerged00:07:24
from beneath the sea and got closer00:07:26
together, some animals were able to00:07:28
island hop across, explaining those00:07:30
early migration pulses. And only around00:07:32
3 million years ago did the ismas appear00:07:34
to close completely, forming a00:07:36
continuous bridge that allowed sloths00:07:38
and many others to wander across freely.00:07:40
Now, it's important to note that this is00:07:41
still an active area of scientific00:07:43
debate. While most researchers agree00:07:44
with this model of the ismas closing00:07:46
gradually over millions of years,00:07:48
they're still arguing about the exact00:07:50
timing of different stages. But this new00:07:51
model helped scientists develop more00:07:53
detailed hypotheses about what happened00:07:55
next and why. Because while scientists00:07:57
had long suspected that the ismas played00:07:59
a role in altering the global climate,00:08:01
this new timeline helped clarify that it00:08:03
was the final closure that had the00:08:05
biggest [music] impact. You see, for00:08:06
millions of years, warm equatorial00:08:08
waters had flowed westward around the00:08:10
globe, including through the seaway00:08:11
between the Atlantic and Pacific. But00:08:13
suddenly, in geological terms at least,00:08:15
that path was blocked, forcing these00:08:17
warm waters to flow north instead. And00:08:19
one hypothesis suggests that this00:08:21
deflection strengthened what we now call00:08:23
the Gulf Stream. Caribbean waters became00:08:25
warmer and saltier. And crucially, all00:08:28
that warm water now flowed toward00:08:29
Greenland and the Arctic. Warmer water00:08:31
meant more evaporation, [music] and more00:08:33
evaporation meant more water vapor in00:08:35
the atmosphere. Vapor that atmospheric00:08:37
circulation was carrying toward the00:08:38
poles. For the first time in millions of00:08:41
years, northern latitude saw enough00:08:43
snowfall that not all of it melted00:08:45
during the summers. And as the snow00:08:46
accumulated year after year, it00:08:48
compacted into ice. By 2.7 million years00:08:51
ago, major glaciation had begun in the00:08:54
northern hemisphere. The ice age had00:08:56
arrived. Now, the ismas definitely00:08:58
wasn't the only factor at play here.00:09:00
Global ice ages are complicated, it00:09:02
turns out. For example, during the00:09:03
period that the ismas was forming, CO200:09:05
levels were also declining, which played00:09:07
a role in the global cooling that had00:09:09
already begun. And because there are00:09:10
competing factors, scientists still00:09:12
debate how much of a role the ismas00:09:14
played in intensifying those global00:09:15
changes. Some scientists have even00:09:17
argued that the ismas actually played no00:09:19
role at all in triggering the ice age,00:09:21
pointing to models that fail to show00:09:22
this as a straightforward cause and00:09:24
effect. The fact that it's been so00:09:26
difficult to pin down the exact00:09:27
chronology of the closure means it also00:09:29
has been really hard to establish causal00:09:30
connections that result from that00:09:32
closure. But while a scientific debate00:09:34
continues, many researchers agree that00:09:36
this narrow strip of land [music]00:09:37
probably played a key role in00:09:39
fundamentally reorganizing Earth's00:09:41
climate. In fact, this tiny piece of00:09:42
land likely shaped the climate we have00:09:44
today. But this geological saga wasn't00:09:46
some sudden crash of continents that00:09:48
suddenly built the ismas. Instead, it00:09:50
was a 12 million-year process of gradual00:09:52
closure, islands rising, channels00:09:55
narrowing, and current squeezing through00:09:56
ever smaller gaps. And when the ismas00:09:58
finally closed completely, [music] this00:10:00
small strip of land barely more than 5000:10:02
kilometers across at its narrowest seems00:10:04
to have had cascading effects across00:10:06
Earth's systems, managing to not only00:10:08
reorganize life on two continents, but00:10:10
also redirecting ocean currents and00:10:12
helping to trigger an ice age, reminding00:10:14
us that sometimes relatively small00:10:17
changes can leave big marks on the00:10:19
planet's history.00:10:24
>> Thank you to Brilliant for supporting00:10:26
PBS. Brilliant is where you learn by00:10:28
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interactive lessons in data analysis,00:10:32
math, programming, and AI. Brilliant00:10:34
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everything on Brilliant. One group of00:11:19
North American animals that flourished00:11:21
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their fascinating evolutionary story was00:11:25
overlooked by Charles Darwin during his00:11:26
famous voyage, [music] which is dumb00:11:28
because his boat was literally named the00:11:30
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Darwin missed an example of evolution00:11:34
right under his nose. Also thanks to00:11:35
this month's eontologists who help00:11:37
connect us.00:11:38
>> Jake Hart, John Davis, Ing, Addie, Carl00:11:42
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Andy, Eric Higgins, Raphael Hassa,00:11:47
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>> Become an Eonite at patreon.com/eons00:11:54
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team. And as always, thanks for joining00:12:00
me in the Andy and Eric Hagen studio.00:12:02
Subscribe at youtube.com/eons00:12:04
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>> I can cry on Q. Do you want me to cry?00:12:15
I'll do it.00:12:16
>> I don't think that's relevant.00:12:17
>> Just give me 45 seconds.00:12:18
>> Can you uncry [laughter]00:12:22
crying right now?00:12:23
>> Yeah.00:12:24
>> Can I [laughter]00:12:25
>> Oh my gosh. Suck them back in. Suck them00:12:27
in.00:12:28
>> Yeah. No, I wasn't.00:12:29
>> It's like a geological fingerprint00:12:31
that's only found in Panama. See, I can00:12:33
turn it on and turn it off just like00:12:34
that. [laughter]Description:
To try everything Brilliant has to offer, visit https://brilliant.org/ On land, the Isthmus of Panama kicked off possibly the greatest natural experiment in the history of life on Earth. In the water, this narrow strip of land did something completely different: it divided. And that division would have a much bigger effect on the planet than the land connection ever did. ***** PBS Member Stations rely on viewers like you. To support your local station, go to https://www.pbs.org/donate/ ***** Eons is a production of Complexly for PBS Digital Studios. Super special thanks to the following Patreon patrons for helping make Eons possible: Nquiztor, lyric1981, SKS PHD, Eric Edwards, Jennifer Courtemanche, Steve Hill, Eric Franklin, raus , Sarah Grunow-Mau, Walter Ray-Dulany, Ruth Orr, Deanna Hernandez, Lianne Lairmore, John Celio, Kevin Lacson, Christopher Samuel, Douglas B, Collin Dutrow, Steven Kern, Nick Ryhajlo, Stephanie Schlea, Betsy Radley, Jeff Graham, Nathan Paskett, Josh L, Lea Nisay, Eric Younge, Elyssa, John D Elias, Annemiek Arkema, Willie, Beth-Ann Cheney, IAmHere, Yibrán Arumir, Sara Lance, Aaditya Mehta, tara thara, Perseus Bluejay, Stephen A Muth III, AllPizzasArePersonal, Hugo van den Hoek, John H. Austin, Jr., Mary Sammartino , Alex Hackman, John Hildebrandt, Melodie Chen-Glasser, Gizmo, Karen Farrell, Jason Rostoker, Mary Tevington, Irene Wood, Derek Helling, CalamityBangs, Brian Clubb, Mark Talbott-Williams, Nomi Alchin, Duane Westhoff, A.B. Heckert, Hillary Ryde-Collins, Yu Mei, Albert Folsom, Dan Caffee, Casey Hague, Susan Freund, William Sunderland, Kerry Conneely, Nicholas Arger If you'd like to support the channel, head over to https://www.patreon.com/eons and pledge for some cool rewards! Want to follow Eons elsewhere on the internet? Facebook - https://www.facebook.com/unsupportedbrowser Instagram - https://www.facebook.com/unsupportedbrowser Bluesky - https://bsky.app/profile/pbseons.bsky.social #Eons References: https://docs.google.com/document/d/1h0E28I5O_nJ7TDaR5aYVU5_4Lmoe30aBaLsadO6J-sA/edit?usp=sharing
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